Need explanation, why does EF core require using attach now

This code works fine month or more
this is HttpPost Api endpoint witch add user to user groups

var user = await _userManager.FindByEmailAsync(model.Email.ToUpper()).ConfigureAwait(false);

// if user not finded or not activated return response

var userUserGroups = await _context.UserGroupsUsers
                    .Where(ugu => ugu.User == user)
                    .Select(w => w.UserGroup)
                    .ToListAsync()
                    .ConfigureAwait(false);

var userGroupsForAdd = promoCodeUserGroups.Where(w => userUserGroups.All(x => x.Id != w.Id)).ToList();

if (!userGroupsForAdd.Any())
{
   //return response
}

await _context.UserGroupsUsers
                    .AddRangeAsync(
                        userGroupsForAdd.Select(
                            w => new UserGroupUser
                            {
                                User = user,
                                UserGroup = w
                            }));

await _context.SaveChangesAsync().ConfigureAwait(false);
//return response all fine

but yesterday started giving an error Cannot insert duplicate key in object 'db.AspNetUsers':

Microsoft.EntityFrameworkCore.Database.Command - Failed executing DbCommand (18ms) [Parameters=[@p0='?' (Size = 450), @p1='?' (DbType = Int32), @p2='?' (Size = 4000), @p3='?' (DbType = DateTime2), @p4='?' (Size = 64), @p5='?' (Size = 4000), @p6='?' (DbType = Boolean), @p7='?' (DbType = Boolean), @p8='?' (DbType = Boolean), @p9='?' (DbType = DateTimeOffset), @p10='?' (Size = 64), @p11='?' (Size = 256), @p12='?' (Size = 256), @p13='?' (Size = 4000), @p14='?' (Size = 4000), @p15='?' (DbType = Boolean), @p16='?' (Size = 4000), @p17='?' (Size = 4000), @p18='?' (DbType = Boolean), @p19='?' (Size = 256)], CommandType='Text', CommandTimeout='30']
SET NOCOUNT ON;
INSERT INTO [cat].[AspNetUsers] ([Id], [AccessFailedCount], [ConcurrencyStamp], [CreatedAt], [Email], [EmailConfirmationCode], [EmailConfirmed], [IsActive], [LockoutEnabled], [LockoutEnd], [Name], [NormalizedEmail], [NormalizedUserName], [PasswordHash], [PhoneNumber], [PhoneNumberConfirmed], [ResetPasswordConfirmationCode], [SecurityStamp], [TwoFactorEnabled], [UserName])
VALUES (@p0, @p1, @p2, @p3, @p4, @p5, @p6, @p7, @p8, @p9, @p10, @p11, @p12, @p13, @p14, @p15, @p16, @p17, @p18, @p19);
System.Data.SqlClient.SqlException (0x80131904): Violation of PRIMARY KEY constraint 'PK_AspNetUsers'. Cannot insert duplicate key in object 'db.AspNetUsers'. The duplicate key value is ({some guid}).

i fixed it.. just using attach for user and usergroups

var isUserInContext = _context.Users.Local.Any(w => w.Id == user.Id);
if (!isUserInContext)
    _context.Users.Attach(user);

foreach (var userGroup in userGroupsForAdd)
{
    var isUserGroupInContext 
        = _context.UserGroups.Local.Any(w => w.Id == userGroup.Id);
    if (!isUserGroupInContext)
        _context.UserGroups.Attach(userGroup);

    var newUserGroupUser 
        = new UserGroupUser
          {
              User = user,
              UserGroup = userGroup
          };

    await _context.UserGroupsUsers.AddAsync(newUserGroupUser);
}

await _context.SaveChangesAsync().ConfigureAwait(false);

but i can not understand why it's work fine before
server moved from windows to linux
but the error started showing up on my windows machine too